Assist/Codelens to show size of type

[TimoFreiberg]

Example:
()<|> -> shows 0
char<|> -> shows 4
struct Foo<|> { ... } shows whatever std::mem::size_of::<Foo>() returns

[bjorn3]

This is going to be very hard to implement, as this basically requires a reimplementation of the layout code in rustc: https://github.com/rust-lang/rust/blob/8ce3f840ae9b735a66531996c32330f24b877cb0/src/librustc_middle/ty/layout.rs is 2776 lines long. It also depends on the exact version of rustc you are using, so it would have to be changed constantly.

[Diggsey]

It also depends on the compilation target...

[TimoFreiberg]

Maybe when the librarification progresses, rust-analyzer and rustc could use the same code?

[matklad]

Yeah, I feel like this is obviously something we should have. More deneraly, we should have an intention to see the whole layout. But it's also true that even MVP version of this would required a lot of progress on modularisation front

[wrenger]

I do miss this a lot. Especially since clangd can show you the size of a struct and the size and offset of its fields.
Also cargo +nightly rustc -- -Zprint-type-sizes or const asserts with size_of are somewhat tedious...

[lnicola]

#12972 is an interesting feature request: it asks for the type of each variant in an enum.

[oxalica]

@bjorn3

This is going to be very hard to implement, as this basically requires a reimplementation of the layout code in rustc: [...] is 2776 lines long

If we only need the size, maintaining (align, size, niche) for each type should be enough. No need for the exact layout and arrangement. It would be much easier.

[bjorn3]

If we only need the size, maintaining (align, size, niche) for each type should be enough. No need for the exact layout and arrangement. It would be much easier.

I don't think it would be much easier. Maybe a bit.

[kyrias]

This would be particularly useful for the implicit futures created by async functions, as per rust-lang/rust-clippy#5263.